[FEATURE] Allow custom filenames/labels for pasted (Ctrl+V) screenshots

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

When pasting a screenshot via Ctrl+V, it's attached with a generic, non-descriptive filename (e.g. image.png) that can't be changed. This causes two concrete problems:

1. Multiple screenshots pasted in the same message (or across a conversation) are indistinguishable by name — there's no way to reference "the second screenshot" or a specific one by an actual identifier in follow-up text.
2. Claude only receives the raw image content, not any upload/paste timestamp metadata. When several visually similar screenshots are pasted (e.g. APM/monitoring traces from a multi-step manual test), Claude has no reliable way to know which screenshot maps to which step — other than guessing from on-screen content or paste order, which can be (and in my case was) wrong.

Concrete example: I ran a 3-round manual test comparing before/after states of a bug fix, took a screenshot after each round, and pasted all three into one message intending to reference them as "Round 1/2/3". Since none could be named, Claude had to guess the mapping from on-screen timestamps and paste order — and got it wrong, because I had actually executed the rounds out of the expected sequence.

Proposed Solution

Allow renaming a pasted image right after it lands in the input area — e.g. clicking the pasted image's thumbnail/chip opens a small inline field to set a custom name (like round2-customerA-vs-B.png). This name should:

• Be visible and editable in the input area before sending.
• Be passed through to the message content as the image's actual filename/label, so it's visible to Claude and can be referenced by name in surrounding text.
• Ideally be combined with #70513's fix, so the custom name is also used for the auto-saved file path.

Alternative Solutions

• Manually typing a label right before/after each pasted image in the same message (current workaround) — works, but easy to forget and doesn't fix the underlying generic-filename problem.
• Pasting each screenshot in its own message with a text label — avoids ambiguity but is cumbersome for batches of related screenshots.
• Exposing real paste/upload timestamps as metadata to the model — would help with ordering, but doesn't solve identification when the user's actual workflow order isn't sequential.

Priority

Medium - Would be very helpful

Feature Category

Developer tools/SDK

Use Case Example

1. Debugging with an APM tool across a sequence of manual test steps (Round 1, 2, 3).
2. After each round, paste a screenshot of the result via Ctrl+V.
3. Before sending, rename each pasted screenshot inline (e.g. round1-guest-control.png, round2-same-group.png, round3-cross-group.png).
4. Reference them by name in the message: "round2-same-group.png shows Customer A got a Miss, Customer B got a Hit."
5. Claude reads the actual filename and matches it unambiguously to the description, instead of guessing from paste order or in-image timestamps.

Additional Context

Environment: Claude Code VS Code extension, screenshots pasted via clipboard (Ctrl+V). Related but distinct from #70513 — even with a stable file path, explicit labels would still be more reliable than an auto-incrementing index or inferred order, especially when steps are executed out of sequence.